Multiple centrifugal or axial pumps are frequently installed in piping systems to increase the overall flow rate to a process (in this case, pumps are operated in parallel), or to increase the overall head produced by the pump combination (pumps are operated in series).
Typically, there is a minimum flow limit to the acceptable flow through a pump. When flow rates are “low,” some pumps experience higher levels of vibration and noise, as well as elevated temperatures (due to low efficiencies). This minimum flow limit is referred to as the Minimum Continuous Stable Flow (MCSF) limit. The level at which vibration or noise becomes unacceptable is specified by the customer, often referring to an industry standard.
Additionally, when pumps are piped in parallel, there may be a range of operation where two flow rates exist for each head value; this occurs when pump performance curves exhibit a point at which the slope is zero when the flow is greater than zero. When two or more pumps are operated in parallel, it is possible for the operating point in a set of pumps to oscillate rapidly between these two flow rates while always maintaining the required head. This rapid change in flow rate can damage or destroy a pump and should be avoided. Many pumps are fitted with recycle or bypass valves for maintaining an adequate flow rate to avoid operating in this hazardous region.
Many pumps are driven with variable-speed drivers such as steam turbines. Varying a pump's speed can be used to control its performance. An alternative is to throttle the discharge valve to maintain performance. When multiple pumps are operated in a network, either parallel or in series, the control objective (usually a flow rate or pressure) can be divided between the pumps in an infinite number of ways.
Present-day speed control systems (for multiple pumps) do not consider the low flow limit. For example, one pump may be running at a high flow rate, while another pump requires an open recycle valve to maintain operation above its MCSF limit. This approach not only increases the risk of a pump operating beyond of its MCSF limit, but it is also inefficient. For these reasons, there is need of a more extensive approach for controlling multiple pumps operating in a network of pumps.